1. Field of the Invention
The present invention concerns a steel suitable for cold processing such as cold forging, particularly, case hardening steel, and method of making the same. The invention also concerns method of making the case hardening steel, especially, for structural use, by continuous casting process.
2. State of the Art
Generally speaking, cold processing of steel is advantageous over hot processing, beause cold processing enjoys not only improved utilization of material due to smaller amount of scrap occurence but also possible reduction of manufacturing cost by automatization and speeding up of the steps. Cold processing further brings about merits of improved accuracy in dimensions of the products and better working surroundings, and therefore, it is being adopted more and more popular in various fields.
In production of machine structural parts such as gears using a machine structural steel as the material, it is usual to prepare the gear by cold processing such as forming by rolling and pressing, and then to strengthen the surface abrasion resistance and the fatigue strength by surface hardening treatment such as carburizing and carbonitriding. At the step of this surface hardening treatment, the blank of gear made by the cold processing is heated to a temperature of austenite domain above A.sub.3 transition point, and held in an atmosphere for surface hardening treatment of the carburizing or the carbonitriding. It has been sometimes experienced that a small member of austenite crystals abnormally grow to form coarse austenite particles of rice-grain size in the steel. Because the coarse particles remain in the steel after hardening and they are more readily hardened in comparison with the surrounding parts, they may cause significant heat treatment strain and decreased resilience.
This problem is particularly significant in case hardening steel for machine structural use. It is experienced even that, further to the decreased resilience due to the coarse particles, crack may occur during the cold processing.
Thus, there has been a demand for the steel for cold forging, which is free from cracking at the cold forging and coarsening of austenite crystals causing decrease of resilience and heat treatment strain during the heat treatment of carburizing or carbonitriding.
In order to prevent the cracking at the cold forging, there has been made efforts to control O-content and S-content so as to minimize oxide inclusion and sulfide inclusion which will provide starting points of the crack at the cold forging. However, it is quite difficult in commercial production of steel to decrease O-content and S-content to such extent that the crack would not occur. Also, decreased amount of S results in lower machinability. As to the coarsening of austenite crystals, no effective solution has been found yet.
In the production of case hardening steel, continuous casting is employed for the purpose of saving energy and stabilizing quality of the products. Recent use of the case hardening steel often requires treatment at such a high temperature as above 1000.degree. C., e.g., vacuum carburizing. Such a high temperature causes coarsening of the austenite crystals of the case hardening steel, and therefore, prevention has been desired.
For the purpose of preventing the coarsening of the austenite crystals at a high temperature, it has been practiced to add Nb and N to the steel so as to precipitate fine Nb(C,N) compounds and to suppress growth of the crystals. Addition amounts of these elements ranges, usually, Nb: 0.03 to 0.06%, and N: 0.008 to 0.014%. However, it is experienced that, in case of continuous casting of the steel containing these elements, the expected effect of suppressing coarsening of the austenite crystals cannot be obtained at the center of the cast piece. Investigation of the cause revealed the phenomenon that Nb(C,N) compounds precipitate in the form of large crystals at the center of the cast piece, where cooling rate is relatively low, and surrounding Nb, C and N concentrate at the center to grow the crystals larger, thus preventing fine Nb(C,N) compounds. The large Nb(C,N) compounds remain in the rolled products as stringer-formed inclusions, which are quite undesirable to some use of the case hardening steel.